One manner of reducing air pollution resulting from the burning of coal is to clean the coal of ash and minerals before burning. Not only is pollution decreased but the efficiency of burning is higher if refuse can be separated from the coal. Moreover, if the sulfur content of coal can be reduced before burning, great benefit is obtained as the emission of sulfur gases resulting from the burning of coal is limited to specified amounts according to law. Thus, it has been an objective of the coal industry to produce a low ash, low sulfur, clean coal through processing.
Different methods and apparatus have been used in pursuit of the objective of producing low ash, clean coal. Most of these methods and apparatus make use of the fact that the physical separation rate of particles in fluid is proportional to the mass of the particle being acted upon, to the magnitude of the the driving force (gravity, magnetic, electrical, etc.) acting upon the particle, and inversely proportional to the resistance of the surrounding fluid to the particle motion and to the distances through which the particle must move to complete the separation.
Among these separation methods is the method of selective flocculation wherein the mass of either the coal or mineral particles is enlarged either magnetically or chemically. Selective flocculation is often carried out in a liquid environment which increases the efficiency of flotation, oil agglomeration, density, magnetic, and most other separations.
Electrostatic separation operates in the air rather than in liquid, and this relatively decreases the resistance term which is inversely proportional to the separation rate. Electrostatic separation rates may also be increased by increasing the driving force (voltage) between the electrodes and ground.
Closely akin to electrostatic separation is electophoretic separation wherein charged particles are suspended in a liquid media. This mechanism is utilized to apply certain coatings to the particles. The resistance term, which was advantageous when the medium was air in the electrostatic separation process, is compensated for by supplying closely spaced electrodes.
Non-aqueous media may be used advantageously to enhance separation. By choosing the liquid used, the resistance (viscosity) of the liquid can be kept small, with the electrical properties thereby being able to increase the driving force. Since organic liquids have much lower heats of vaporization than water, the processing in non-aqueous media may be advantageous if a dried product is needed.
A disc or a decanter centrifuge may be useful for fine coal cleaning if a fluid is used which has a density between that of coal and the mineral matter (refuse). The centrifuge permits much higher forces than allowed by gravity or cyclone separations, and the closely spaced discs of the disc centrifuge permit the particles to separate without moving a great distance. One drawback of the disc centrifuge, however, is that the coal input must be very fine so as to avoid plugging and erosion.
Various flotation methods such as electroflotation, dissolved air flotation, air-sparged cyclones etc. have also been proposed. These methods emphasize a close contact between particle and newly forming bubbles. The air-sparged cyclone also features a very short distance of travel for the loaded bubble between its origin and overflow.
All of the various above listed methods, as well as other techniques such as chemical cleaning processes have different benefits depending on the degree of cleaning and the make-up of the end product required. However, all suffer from various technological difficulties as well as other well known drawbacks. Additionally, as many of the methods compete, the cost of the different methods for cleaning fine coal is an important consideration.
It is therefore, an object of the invention to provide a novel method for the removal of mineral matter and ash from fine particle coal in an economical manner.
A further object of the invention is to increase the weight fraction of fine coal recovery while decreasing the ash and sulfur content by pretreatment of the slurry of coal immersed in heavy liquid prior to centrifugal separation.
Yet a further object of the invention is to provide an improved method for the cleaning of fine coal whereby the slurry of coal immersed in heavy liquid is pretreated by both the addition of a surfactant and the use of ultrasonic dispersion prior to centrifugal operation.
It is even another object of the invention to provide an improved method for the cleaning of fine coal where the coal and refuse particles are all less than 0.075 mm. in size and where a coal heavy liquid slurry is pretreated prior to a centrifugal separation.